Efforts are being made to achieve exposure of heat-developable light-sensitive materials with low-irradiance light sources such as LED, CRT, FOT and semiconductor lasers. Research is also being undertaken with a view to shortening the time required for image formation. In particular, light-sensitive materials such as thermally developable ones which are adapted for rapid access are the subject of extensive studies including efforts to produce high-speed heat-developable light-sensitive materials which require shorter exposure times.
Heat-developable light-sensitive materials can be rendered highly sensitive by increasing the content of silver iodide in the light-sensitive silver halide but, as it turns out, the increased silver iodide content causes enhanced thermal fogging. In order to suppress thermal fogging, the use of thermal fog preventing agents has been proposed and compounds included within this class are: the mercury compounds shown in U.S. Pat. No. 3,589,903; the N-halogeno compounds shown in West German Patent No. 2,402,161; the peroxides shown in West German Patent No. 2,500,508; the sulfur compounds shown in West German Patent No. 2,617,907; the palladium compounds shown in U.S. Patent No. 4,102,312; the sulfinic acids shown in Japanese Patent Publication No. 28417/1978, the mercaptotetrazole compounds shown in Research Disclosure Nos. 169077 and 169079; and the 1,2,4-triazole shown in U.S. Pat. No. 4,137,079. However, none of these antifoggants are completely satisfactory for various reasons such as high toxicity to humans and low efficacy.
The present inventors made concerted efforts to solve these problems and, as a result, they found that a heat-developable light-sensitive material that satisfies both the requirements for high sensitivity and small thermal fogging can be attained by employing a silver halide emulsion that comprises light-sensitive silver halide grains of the core/shell type that contain a specified amount of silver iodide and which have a lower silver iodide content in the surface layer than in the bulk or internal phase.